#697 fix Random seam position when only one perimeter.

fix random for thin wall loops (less than one perimeter)

src/libslic3r/ExtrusionEntity.hpp

@@ -88,6 +88,8 @@ enum ExtrusionLoopRole : uint16_t {
     elrHole = 1 << 3, // 8
     //it's a modifier that indicate that the loop should be printed as vase
     elrVase = 1 << 4, //16
+    //it's a modifier that indicate that the loop does not contains an inner loop, used for random seam
+    elrFirstLoop = 1 << 5, //32
 };
 
 

src/libslic3r/GCode/SeamPlacer.cpp

@@ -206,6 +206,8 @@ void SeamPlacer::init(const Print& print)
        explgs = Slic3r::offset_ex(explgs, scale_(max_nozzle_dmr));
    for (ExPolygons& explgs : m_blockers)
        explgs = Slic3r::offset_ex(explgs, scale_(max_nozzle_dmr));
+
+   this->external_perimeters_first = print.default_region_config().external_perimeters_first;
 }
 
 
@@ -439,22 +441,30 @@ Point SeamPlacer::get_seam(const Layer *layer, SeamPosition seam_position,
         return polygon.points[idx_min];
 
     } else { // spRandom
-        if ( (loop.loop_role() & elrInternal) != 0 && loop.role() != erExternalPerimeter) {
+        if ( (loop.loop_role() & ExtrusionLoopRole::elrInternal) != 0 && loop.role() != erExternalPerimeter) {
             // This loop does not contain any other loop. Set a random position.
             // The other loops will get a seam close to the random point chosen
             // on the innermost contour.
-            //FIXME This works correctly for inner contours first only.
             last_pos = this->get_random_seam(layer_idx, polygon);
-        }
+        } else if (loop.role() == erExternalPerimeter) {
-        if (loop.role() == erExternalPerimeter && is_custom_seam_on_layer(layer_idx)) {
+            if (is_custom_seam_on_layer(layer_idx)) {
-            // There is a possibility that the loop will be influenced by custom
+                // There is a possibility that the loop will be influenced by custom
-            // seam enforcer/blocker. In this case do not inherit the seam
+                // seam enforcer/blocker. In this case do not inherit the seam
-            // from internal loops (which may conflict with the custom selection
+                // from internal loops (which may conflict with the custom selection
-            // and generate another random one.
+                // and generate another random one.
-            bool saw_custom = false;
+                bool saw_custom = false;
-            Point candidate = this->get_random_seam(layer_idx, polygon, &saw_custom);
+                Point candidate = this->get_random_seam(layer_idx, polygon, &saw_custom);
-            if (saw_custom)
+                if (saw_custom)
-                last_pos = candidate;
+                    last_pos = candidate;
+            } else if (external_perimeters_first || (loop.loop_role() & ExtrusionLoopRole::elrFirstLoop) != 0) {
+                // this is if external_perimeters_first
+                // this is if only space for one externalperimeter.
+                //in these case, there isn't a seam from the inner loops, so we had to creat our on
+                last_pos = this->get_random_seam(layer_idx, polygon);
+            }
+        } else if (loop.role() == erThinWall) {
+            //thin wall loop is like an external perimeter, but without anything near it.
+            last_pos = this->get_random_seam(layer_idx, polygon);
         }
         return last_pos;
     }

src/libslic3r/GCode/SeamPlacer.hpp

@@ -52,6 +52,9 @@ private:
     //std::map<const PrintObject*, Point>  m_last_seam_position;
     SeamHistory  m_seam_history;
     
+    // if it's expected, we need to randomized at the external periemter.
+    bool external_perimeters_first;
+
     // Get indices of points inside enforcers and blockers.
     void get_enforcers_and_blockers(size_t layer_id,
                                     const Polygon& polygon,

src/libslic3r/PerimeterGenerator.cpp

@@ -1103,6 +1103,9 @@ ExtrusionEntityCollection PerimeterGenerator::_traverse_loops(
         if (!loop.is_contour) {
             loop_role = (ExtrusionLoopRole)(loop_role | ExtrusionLoopRole::elrHole);
         }
+        if (loop.children.empty()) {
+            loop_role = ExtrusionLoopRole(loop_role | ExtrusionLoopRole::elrFirstLoop);
+        }
         if (this->config->external_perimeters_vase.value && this->config->external_perimeters_first.value && is_external) {
             if ((loop.is_contour && this->config->external_perimeters_nothole.value) || (!loop.is_contour && this->config->external_perimeters_hole.value)) {
                 loop_role = (ExtrusionLoopRole)(loop_role | ExtrusionLoopRole::elrVase);